Reduced-pH granular detergent compositions are known, and can have a pH of less than about pH 10. Low pH compositions offer the advantage of being less harsh to skin, reduced color fading, and in certain cases, improved stain removal. Examples of such compositions are disclosed in Japanese Patent Laid-Open S54-160405, (Ajinomoto), Dec. 19, 1979; Japanese Patent Laid-Open S54-149707, (Lion) Nov. 24, 1979; German Patent publication 2,559,631 (Henkel) May 18, 1977; U.S. Pat. No. 4,707,287, (Herdeman) Nov. 17, 1987; Japanese Patent Laid-Open S62-4794, (Kao Soap) Jan. 10, 1987; U.S. Pat. No. 4,170,453, (Procter & Gamble Company) Oct. 9, 1979; U.S. Pat. No. 4,810,413, (Pancheri et al) Mar. 7, 1989; GB Patent 2,106,482 (Kaeser); and Mexican Patent 172,329, (Leslie et al) Dec. 13, 1993.
To achieve the reduced pH of the compositions of the above references, it is disclosed to utilize, from among a variety of acidic materials, a weak acid material such as citric acid, or a half-salt such as sodium acid pyrophosphate. Both of these ingredients also serve as a builder; that is, a material that can sequester calcium and magnesium ions (often called "hardness") in the wash water.
Acid pyrophosphate can form a hydrate, and lose this water of hydration at a temperature above about 80.degree. C. For this reason, it is preferred to avoid exposing the acid pyrophosphate hydrate to temperatures above 80.degree. C., such as to temperatures achieved routinely in a conventional spray-drying operation, which are well known to those skilled in the art. In such operation, detergent ingredients, both liquid and dry forms, are formed together into a slurry which is then introduced into a counter-current spray-drying tower, thereby forming a spray-dried product.
Alternatively, such acid builder materials are preferentially admixed with spray-dried granules. More preferably, however, such acid builders are processed into a product using only low-temperature agglomeration processes. Such processes include the V-blender process disclosed in WO-92-6170 (The Procter & Gamble Company), Apr. 16, 1992, the disclosure of which is incorporated herein by reference. Such processes can also include agglomeration processes using other well known equipment such as the Littleford mixer or Lodige mixer, as described in co-owned and co-pending applications U.S. Ser. No. 08/92048 (Capeci et al.), filed Oct. 15, 1993, Attorney Docket No. 5043; U.S. Ser. No. 08/137,877 (Pancheri), filed Jul. 15, 1993, Attorney Docket No. 4952; and U.S. Ser. No. 08/83,145 (Welch et al.), filed Jun. 25, 1993, Attorney Docket No. 4921. Such processes provide intimate incorporation of the acid builder into the base granular material (that is, the material comprised of the detergent surfactant and optionally other detergent builders).
However, such disclosures, while teaching the preferred use of citric acid as the acid builder, fails to recognize the deliquescent nature of citric acid which is detrimental to the physical stability of the product containing the citric acid, particularly in hot and humid conditions which are common in most parts of the world. By deliquescent is meant that the citric acid forms a hydrate from moisture and then proceeds to dissolve in its own water of hydration. In granular detergent products, if such deliquescing occurs when the citric acid is admixed, or agglomerated, with the base granular material, the product becomes lumpy, cakey, and pours poorly, if at all.
The present invention then is the discovery that substantially improved low pH detergent products can be formed by using a low acidic material or a half-acid material, preferably a builder, such as acid pyrophosphate, and that maintains a stable hydrate below temperatures of about 90.degree. C., and by avoiding or minimizing the use of such acid builders which have hydrates that are not stable below about 50.degree. C., or which are deliquescent, such as citric acid. Such products have substantially improved physical properties and, when formed using an agglomeration process, resist segregation of the acid builder material from the base detergent material.